CEL212 Environmental Engineering

Transcription

1 Page 1 Indian Institute of Technology Delhi CEL212 Environmental Engineering Second Semester (Dr. Arun Kumar; arunku@civil.iitd.ac.in) Laboratory 7: Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) 7A: Biochemical Oxygen Demand (BOD) Objective : To determine BOD value for determining biodegradability of solution. Background: The most widely used test indicating organic pollution of both wastewater and surface water is the 5-day BOD (BOD 5 ). This determination involves the measurement of the dissolved oxygen used by microorganisms in the biochemical oxidation of organic matter. BOD 5 is the total amount of oxygen consumed by microorganisms during the first five days of biodegradation. Oxygen demand is associated with the biodegradation of the carbonaceous portion of wastes and oxidation of nitrogen compounds such as ammonia. The following equations simplify the process of biodegradation: Organic matter + O 2 + microorganisms CO 2 + H 2 O + new microbial cells Ammonia + O 2 + microorganisms NO 3 + H 2 O + new microbial cells Procedure: Apparatus: Incubation bottle 300mL volume; Air compressor, 20 C incubator Reagents for DO measurement: 1. Manganese sulfate solution: Dissolve 480 g MnSO 4.4H 2 O, 400 g MnSO 4.2H 2 O or 364 g MnSO 4.H 2 O in distilled water, filter, and dilute to 1L. The MnSO 4 solution should not give a color with starch when added to an acidified potassium iodide (KI) solution. 2. Alkali-iodide-azide reagent 3. Sulfuric acid: One ml is equivalent to ~ 3mL alkali-iodide-azide reagent. 4. Starch solution: Dissolve 2 g laboratory-grade soluble starch and 0.2 g salicyclic acid as preservative in 100 ml hot distilled water. 5. Standard sodium thiosulfate titrant: Dissolve g Na 2 S 2 O 3. 5H 2 O in distiller water and add 1.5 ml 6N NaOH or 0.4 g solid NaOH and dilute to 1000 ml. Standardize with biiodate solution. 6. Standard potassium bi-iodate solution (0.0021M): Dissolve mg KH(IO 3 ) in distilled water and dilute to 1000 ml. 7. Standardization: Dissolve e ~ 2 g KI, free from iodate in an Erlenmeyer flask with 100 to 150 ml distilled water; add 1 ml 6N H 2 SO 4 or a few drops of conc. H 2 SO 4 and ml standard bi-iodate solution. Dilute to 200 ml and titrate librated iodine with thiosulfate titrant, adding starch toward end of titration, when a pale straw color is reached. When the solution is of equal, ml 0.025M Na 2 S 2 O 3 should be required. If not, adjust the Na 2 S 2 O 3 solution to 0.025M.

2 Page 2 Steps: DO measurement: 1. Make dilution water by adding 2mL/L of following reagents in distilled water: a. Phosphate buffer solution b. Magnesium sulfate solution c. Calcium chloride solution d. Ferric chloride solution e. Sodium Sulfite solution 2. For a given sample bottle, add 1 ml of alkali azide and then 1 ml manganous sulfate solution. Shake well the bottle and keep it open for 5 minutes to settle the precipitate. Add 2 ml concentrated H 2 SO 4 and place the cap on the bottle. Shake well the bottle till all the precipitate is dissolved. 3. Take 203 ml of sample in conical flask and titrate with standard sodium thiosulfate solution (0.025N) till the colour changes from dark yellow to light yellow. Then add few drops of starch indicator and continue to titrate till the color of the solution becomes either colorless or changes to its original sample colour. Note down volume of 0.025N sodium thiosulfate consumed. 4. Calculate DO value of the sample. Remember that in 200 ml sample, 1 ml of sodium thiosulfate of 0.025N equals to 1 mg/l dissolved oxygen: =>Dissolved oxygen (DO) (in mg/l) = ml of sodium thiosulfate (0.025N) consumed. BOD: 1. Prepare BOD dilutions. Use dilution water (it contains nutrients, the exact contents are described in Standard Methods):Blank (only dilution water);5 ml sample in 300 ml BOD bottle, fill up with dilution water;15 ml sample in 300 ml BOD bottle, fill up with dilution water;20 ml sample in 300 ml BOD bottle, fill up with dilution water 2. Take 300 ml sample in BOD bottle. Prepare two sets of this sample. Keep one set for DO analysis for day 0 (i.e., Sample0Day) and another sample in BOD incubator for 5 days at 20 C (Sample5Day). 3. Measure DO in different samples at t=0. 4. Incubate samples in 20 o C for 5 days. 5. Come back in the lab after 5 days and record dissolved oxygen. 6. Record data in following manner. Bottle no. Wastewater sample (ml) Initial DO (mg/l) (DO 0 ) Calculate 5-day BOD value of the sample at 20 C: t-day BOD= [DO t -DO 0 ]/(P) (1) where P= Dilution factor = 300mL/(sample volume in ml) DO at 5-day (ml) (DO 5 ) Questions to Answer: 1. If 5-day BOD at 20 C=150mg/L (k=0.23/day), calculate 5-day BOD at 15 C and 22 C? Use following dependence of (k) with temperature: k T =k 20 {θ} (T-20)

3 Page 3 Reference: AWWA, WEF, APHA, 1998, Standard Methods for the Examination of Water and Wastewater (Method: 4500-O. C. Azide Modification) AWWA, WEF, APHA, 1998, Standard Methods for the Examination of Water and Wastewater (Method: 5210B,5-day BOD) Sawyer, C.N., McCarty, P.L., and Parkin, G.F Chemistry for Environmental Engineering 4 th Edition. Tata McGraw-Hill Publishing Company Limited Additional questions (not to submit) Q: Calculate oxygen consumption rates of samples with given data and comment on biodegradability time(d) sample 1(BOD(mg/L) at 20degC) sample 2(BOD(mg/L) at 20degC)

4 Page 4 7B:Chemical Oxygen Demand (COD) (closed reflux method) Objective 2: To determine COD value for determining organic strength of solution (Closed Reflux Method) Background: Chemical oxygen demand (COD) is termed as the amount of a specific oxidizing agent that reacts with sample under controlled conditions and it is expressed as oxygen equivalence. This parameter indicates the extent of organic matter contamination of water and is always higher than the biochemical oxygen demand (BOD). It is used to indicate organic matter contamination and it helps in knowing overall organic load to the receiving body. Selection of Method There are two methods for COD determination. The first method: open reflux method is suitable for a wide range of wastes where large volume of sample is required (for samples with COD= 50 mg O 2 /L). In the second method: closed reflux methods, small quantities of metallic salt reagents are required and small quantities of hazardous waste is produced (for samples with COD= 5 to 50 mg O 2 /L). In the closed reflux method, ampules and culture tubes with premeasured reagents are used and then samples is placed in the tube and COD is determined. In this experiment, closed reflux method is used and samples with COD < 50 mg O 2 /L are tested. Reaction with dichromate solution of sample: Potassium dichromate is a strong oxidizing agent and it can be used to prepare solution of exact normality. C n H a O b N c +d Cr 2 O (8d+c) H + => nco 2 +[(a+8d-3c)/2]h 2 O+c NH dCr 3+ (1) Here d=(2n/3)+(a/6)-(b/3)-(c/2) During experiment, excess dichromate concentration is determined by titrating it with ferrous ammonium sulfate (FAS). The reaction is given by: 6Fe 2+ Cr 2 O H + => 6Fe Cr 3+ +7H 2 O (2) Here d=(2n/3)+(a/6)-(b/3)-(c/2) Ferroin (ferrous 1,10-phenanthroline sulfate): It is used to indicate change in oxidation-reduction potential of the solution and it indicates the condition when all dichromate has been reduced by ferrous ion. It gives a very sharp brown color change which can be seen in spite of blue color generated by the Cr 3+ ions formed on reduction of the dichromate. Procedure: Apparatus: Digestion vessels; block heater; microburet; ampule sealer. Borosilicate culture tubes (16mm*100 mm or 20 mm*150mm) with TFE lined-screw caps are used. The block heater is required to operate at 150±2 C with holes to accommodated digestion vessels. Do not use an oven because of the possibility of leaking samples generating corrosive and explosive atmosphere. Reagents:

5 Page 5 a. Standard potassium dichromate digestion solution, M: Add to about 500 ml distilled water g K 2 Cr 2 O 7, primary standard grade, previously dried at 150 C for 2 h, 167 ml conc. H 2 SO 4, and 33.3 g HgSO 4. Dissolve, cool to room temperature, and dilute to 1000 ml. b. Sulfuric acid reagent: c. Ferroin indicator solution: Dilute it by a factor of 5 as required. This indicator is used to indicate change in oxidation-reduction potential of the solution. d. Standard ferrous ammonium sulfate titrant (FAS), approximately 0.10M: Dissolve 39.2 g Fe(NH 4 ) 2 (SO 4 ) 2.6H 2 O in distilled water. Add 20 ml conc H2SO4, cool, and dilute to 1000 ml. Standardize solution daily against standard K2Cr2O7 digestion solution as follows: Pipet 5.00 ml digestion solution into a small beaker. Add 10 ml reagent water to substitute for sample. Cool to room temperature. Add 1 to 2 drops diluted ferroin indicator and titrate with FAS titrant. Molarity of FAS solution = [V K2Cr2O7 0.1] / (V FAS ) (3) Where: V K2Cr2O7 = volume of K2Cr2O7 (ml); V FAS = volume of FAS (ml) e. Sulfamic acid: f. Potassium hydrogen phthalate standard: Steps: 1. Wash culture tubes and caps with 20% H2SO4 before using to prevent contamination. 2. Place sample in culture tube or ampule and add digestion solution. Carefully run sulfuric acid reagent down inside of vessel so an acid layer is formed under the sample-digestion solution layer and tightly cap tubes or seal ampules, and invert each several times to mix completely. 3. Place tubes or ampules in block digester preheated to 150 C and reflux for 2 h behind a protective shield. CAUTION: These sealed vessels may be under pressure from gases generated during digestion. Wear face and hand protection when handling and dangerous pressures will be generated at 150 C. 4. Cool to room temperature and place vessels in test tube rack. Some mercuric sulfate may precipitate out but this will not affect the analysis. 5. Remove culture tube caps and add small TFE-covered magnetic stirring bar. If ampules are used, transfer contents to a larger container for titrating. 6. Add 0.05 to 0.10 ml (1 to 2 drops) ferroin indicator and stir rapidly on magnetic stirrer while titrating with standardized 0.10M FAS. The end point is a sharp color change from blue-green to reddish brown, although the bluegreen may reappear within minutes. In the same manner reflux and titrate a blank containing the reagents and a volume of distilled water equal to that of the sample. 7. COD is given by COD as mg/l O 2 /L = [(A-B) M 8000) / (V sample ) (4) Where: A = volume of FAS used for blank (ml); B= volume of FAS used for sample (ml); M=molarity of FAS; 8000= miliquivalent weight of oxygen 1000 ml/l Analyze samples in duplicates because of small sample size. Questions to Answer: 1. Why do the COD analysis and BOD analysis give different results for the same waste?

6 Page 6 2. What could be inferred from the following samples concerning the relative ease of biodegradability: Sample A (5-d BOD/COD=24/30) and Sample B (5-d BOD/COD=10/50)? Reference: AWWA, WEF, APHA, 1998, Standard Methods for the Examination of Water and Wastewater (Methods: 5220 C. Closed Reflux Titrimetric Method) Sawyer, C.N., McCarty, P.L., and Parkin, G.F Chemistry for Environmental Engineering 4 th Edition. Tata McGraw-Hill Publishing Company Limited.